Fiber reinforced, plastic base materials are widely used as insulating support for copper conductive patterns of printed wiring boards. Printed wiring boards are manufactured by subtractive and by additive processes. In subtractive processes, the conductive pattern is obtained by selective removal of unwanted portions of a conductive foil. In fully-additive processes, the entire thickness of electrically isolated conductors is built up by electroless metal deposition. Base materials are manufactured for fully-additive processes which contain a catalyst for electroless metal deposition supported on a clay filler. The FR-4 type, additive base material contains 12 parts of palladium catalyzed clay filler per hundred parts of epoxy resin.
Among the thermosetting polymers used as printed wiring base materials are phenolic, epoxy, polyimide, cyanate ester and bismaleimide triazine resins. In soldering operations, printed wiring boards experience temperature excursions from 20.degree. C. to 300.degree. C., which severely stress the metallic conductive pattern. Woven fiber reinforcements anisotropically restrict thermal expansion in the warp and woof directions, so that most of the expansion is along the Z-axis. Glass cloth is the common fiber reinforcement in printed wiring boards, but woven aramid fiber and woven quartz fiber reinforcement have been used to further reduce the thermal expansion in the warp and woof directions. This reduced X-axis and Y-axis expansion increases the thermal expansion in the Z-axis direction.
The thermal expansion along the Z-axis of the glass cloth reinforced epoxy laminates from room temperature to soldering temperature is typically 10 to 20 times greater than the thermal expansion of copper along the Z-axis. Between 30.degree. and 270.degree. C., conventional glass cloth reinforced epoxy laminates expand about 5% along the Z-axis while the copper on the wall of a plated-through hole expands only 0.4%. This imposes a severe stress on the copper in the plated-through holes. Copper on the walls of plated-through holes in thin base materials (less than 0.8 mm thick) usually withstands the stress. In thicker materials (e.g., 1.6-3.2 mm thick) the strain causes cracks or even complete breaks in the copper conductors of the plated-through holes.
High ductility copper deposits are required to reduce or prevent cracks and breaks in plated-through holes. Copper electroplating solutions that will provide copper deposits capable of 15-20% elongation are recommended in the manufacture of subtractive printed wiring boards with plated-through holes. If the ability to provide deposits with 15-20% elongation is maintained, the failure rate of the printed wiring boards is only about 0.2%. However, in the manufacturing environment such copper electroplating solutions are subject to frequent upsets where failure rates of plated-through holes go to 25-100%. For fully additive, printed wiring boards with plated-through holes, electroless copper plating solutions providing copper deposits capable of 6-10% elongation have been recommended. It also has proven to be difficult to maintain the capability to deposit copper with this high elongation in a daily manufacture of additive wiring boards.
Most polymer systems used in base materials for printed wiring boards undergo a change in the rate of thermal expansion at a glass transition temperature, T.sub.g. The T.sub.g for standard epoxy base materials is 110-130.degree. C.; for high T.sub.g epoxies, it is 140-175.degree. C.; for bismaleimide triazines, it is 160.degree. C.; for cyanate esters, it is 220-250.degree. C., and for polyimides, it is 240-290.degree. C. Below the T.sub.g the Z-axis coefficient of thermal expansion, CTE, is in the range of 50-100.times.10.sup.-6 /.degree.C. Above the T.sub.g the Z-axis CTE is in the range of 200-300.times.10.sup.-6 /.degree.C. It is possible to reduce the thermal stress by utilizing polymer systems with a higher T.sub.g which provide a lower average CTE over the temperature range of 30-270.degree. C. Plated-through holes in glass cloth reinforced polyimide laminates with high T.sub.g have been shown to resist thermal stress cracking, while plated-through holes produced under the same conditions in glass reinforced epoxy laminates fail in thermal stress. These higher T.sub.g materials are more expensive, more difficult to process, especially in drilling and metallizing plated-through holes, and, with the exception of polyimides, have not clearly demonstrated reduced failures in plated-through holes under thermal stress.
In the prior art, the CTE along the Z-axis of a FR-4 epoxy glass base material has not been closely controlled. The typical CTE from 30.degree. C. to 270.degree. C. of FR-4 epoxy glass laminates has averaged about 260 ppm and has varied from 210 to 320 ppm. Likewise, the percent elongation of the copper deposited has been difficult to precisely control. Since the CTE above T.sub.g is approximately 400% greater than the CTE below T.sub.g these average CTEs decrease somewhat as the T.sub.g increases. Other factors that contribute to variable CTE along the Z-axis are type of glass cloth, the resin pickup on the glass cloth, and variation in the press cure cycle and position of the individual sheets of base material in the press or book. Experience has shown that while the variation of the CTE along the Z-axis within one production lot of base material has been relatively small, it has been as much as 60 ppm from lot to lot of presumably identical material.
Composite Epoxy Materials (CEM) are fiber reinforced laminates using two or more types of reinforcing material. CEM laminates have not been widely used. The total area of double sided, copper clad, CEM laminates consumed in 1990 was only 5% of FR-4, glass cloth reinforced epoxy laminate consumption. The most widely used Composite Epoxy Material is CEM-3. CEM-3 is made with two woven glass face sheets, a nonwoven glass core, and a flame resistant epoxy resin. Typical CEM-3 construction is 4-8 sheets of epoxy impregnated glass mat or glass paper with woven glass prepregs as face sheets on each side. Through holes in FR-4 laminates must be drilled, but CEM laminates are designed to be punchable so that holes can be formed by lower cost punching techniques. However punching of CEM-3 laminates has been difficult because die clearances must be maintained at 25-50 micrometers instead of the 50-75 micrometers normally used in printed wiring board fabrication. Also, the punches must be resharpened more frequently. CEM-3 laminates warp and twist properties, impact strength and flexural strength are inferior to those of FR-4 laminates. Since the glass paper layers used in CEM-3 have less dimensional stability than the woven glass cloth used in FR-4, CEM-3 has less stability in the X- and Y-axes and should have less expansion in the Z-axis than FR-4. Because of its deficiencies, CEM-3 has not been adequate for use in the high quality markets served by woven cloth laminates such as FR-4.
Japanese Laid Open Patent Application 222,950 of 1989 has the goal to improve the molding of CEM-3 laminates by reducing the known deficiencies of (1) laminates which are thinner at the edges and thicker in the middle, and (2) poor adhesion between the surface prepreg layers of woven glass and the intermediate prepreg layers of non-woven glass. It describes copper clad, CEM-3 laminates which are made with an epoxy varnish containing 10-250% by weight of needle-shaped, inorganic fillers such as wollastonite. The filler was added to reduce the "squeeze out" of resin near the edges while permitting use of more fluid resin in the non-woven prepregs for better layer to layer adhesion. The filler was optionally treated with a silane coupling agent.
Japanese Laid Open Patent Applications 199,643 of 1982 and 7,044 of 1984 propose methods to improve the deficiencies in punchability and the X- and Y-axis dimensional stability of CEM-3 laminates. They describe copper clad, CEM-3 laminates that contain 5-50% wollastonite or 5-50% potassium titanium oxide plus 1-30% other fillers. Even with the improved dimensional stability, these laminates had a CTE in the Z-axis of 180.times.10.sup.-6 /.degree.C.
To overcome the poor punching qualities and moldability of CEM-3 laminates, copper clad, CEM-3 laminates described in Japanese Laid Open Patent Application 97,633 of 1989 contain a third sheet of woven glass in the center of the unwoven fiber reinforcement along with boehmite and/or gibbsite, aluminum hydroxide filler, 80-200% by weight based on the resin content and a high temperature bisphenol A novolac epoxy resin. These CEM-3 laminates are reported to have improved dimensional stability, solder resistivity and plated-through hole reliability compared to conventional CEM-3 laminates.
Japanese Laid Open Patent Application 120,330 of 1990 also describes copper clad, nonwoven glass fiber laminates where 50-75% by weight aluminum hydroxide filler is added to a mixed epoxy and phenolic resin varnish in order to overcome some of the deficiencies of CEM-3 with regard to punching and dimensional stability in the Y-axis.
Even with these proposed improvements the properties of the CEM-3 laminates still do not permit their substitution in the markets served by FR-4 laminates.
Conventional epoxy laminates contain epoxy resins having a tetrabromophenyl group to provide fire retardance. When the laminate is exposed to flame or high temperature, bromine is released, which retards burning. The brominated resin laminates are more difficult to process than non-fire retardant laminates made with epoxy resins without bromine. Japanese Patent Application 117,912 of 1990 proposes the addition of an aluminum hydroxide filler to a non-brominated epoxy resin in woven glass cloth reinforced laminates to make a fire retardant laminate. Since like brominated resins, aluminum hydroxide also decomposes at high temperatures (releasing water above 200.degree. C.), it provides fire retardance. Fifty percent by weight filler was added to a varnish; woven glass cloth was impregnated with the varnish, and eight sheets laminated together to make a flame retardant laminate. The laminate was reported to have good soldering heat resistance and measling resistance. The disadvantage of aluminum hydroxide filler in the laminate is the decomposition of the filler at elevated temperatures which can affect Z-axis expansion.
Silane coupling agents and organosilicon chemicals are used to upgrade the physical and electrical properties of mineral and glass filled thermoplastic resin systems to values approaching or sometimes exceeding the unfilled resin. In thermosetting resins, silane coupling agents are applied to the woven glass cloth in FR-4 laminates to improve the bond between the epoxy resin and the woven glass cloth. Silane coupling agents also have been used on alumina trihydrate in epoxy molding compounds to improve the electrical properties of the molded part. However, silane coupling agents are subject to hydrolysis and are not suitable for base materials which will be subjected to prolonged exposure to alkaline solutions as in some printed wiring board processing procedures.
Titanate and zirconate coupling agents react with an inorganic filler to make it hydrophobic, organophilic and organofunctional. In thermoplastic systems, filled polymers containing titanate and zirconate treated fillers have lower melt viscosity and are more easily processed than filled polymers without coupling agents. In thermosetting epoxy moldings filled with milled aramid, moldings treated with titanate coupling agents had triple the flexural and impact strength compared to moldings without coupling agents. Likewise, epoxy systems filled with potassium titanate whiskers had improved impact strength when titanate coupling agents were used. However, the use of organotitanate and organozirconate coupling agents has not gained acceptance in printed wiring board base materials.